Bone Morphogenetic Proteins (BMPs) are pleiotropic growth factors that have multiple functions in mammalian gametogenesis including primordial germ cell (PGC) generation, spermatogonia proliferation, and meiotic germ cell survival. Our preliminary studies have shown that several murine genes encoding BMPs of the DPP class (BMP2 and BMP4) and the 60A class (BMP7, BMP8A, and BMP8B), BMP receptors (ALK3 and TALK), and the downstream signal transducer SMAD1 are expressed in the developing and adult testes. Mutations in several Bmp genes result in spermatogenic defects. Bmp genes within the 60A class have additive effect, while such additive effect has not been observed between the 60A and Dpp classes. These findings support a notion that the 60A and DPP classes may have distinct roles by binding to separate receptor complexes on testicular cells in vivo. This postulate will be further tested in Specific Aim 1 by fully examining the mutant phenotype of Bmp4 heterozygotes in inbred strains and by rescuing Bmp4 mutants with Bmp8a transgene. Alk3 and Smad1 are expressed in spermatogonia and spermatocytes, suggesting that they may play roles by transducing BMP signaling. This hypothesis will be tested by the newly developed Cre/lox recombination system to inactivate both genes specifically in male germ cells. Therefore, the roles of Alk3 and Smad1 in spermatogenesis will be revealed without affecting other developmental systems including PGC development. Moreover, both BMP8B and C-KIT signaling pathways play critical roles in PGC development and spermatogenesis, and their mutant phenotypes are strongly affected by genetic backgrounds. Thus, Bmp8b and W mutants will serve as excellent models for the identification, mapping, and cloning of genetic modifiers that influence or interact with BMP8B and C-KIT signaling during PGC development as well as in spermatogenesis. One major modifier has been discovered for each mutant through genetic testing. As described in Specific Aim 2, efforts will be made to map and eventually clone these modifiers for their roles in germ cell development by using a combination of genetic mapping and molecular cloning strategies. Therefore, the successful execution of this proposal with the inter-related two aims will add significantly to our understanding of the molecular mechanisms controlling mammalian gametogenesis.